鑽石膜之內應力機制與光學性質

Abstract

本實驗採用熱燈絲化學氣相沉積法（HFCVD），以CH4/H2為反應氣體 （流量100sccm），變化甲烷濃度，沉積時間，沉積溫度及基材前處理在 P-type（100）矽基板上沉積鑽石膜。藉由量測FTIR穿透光譜並以不同的 模型進行數學模擬來分析鑽石膜之光學性質。沉積膜以SEM，Raman，XRD 分析其特性，並利用拉曼光譜法及X-射線繞射法計算鑽石膜內應力值，配 合Rietveld精算法修正其晶格常數值，藉以比較各種殘留映力估計法，以 探討鑽石膜內應力機制及其與光學性質之關係。 實驗結果顯示，鑽石膜殘留應力的主要來源為熱應立即本質應力。對鑽石 膜/矽基材而言，X-射線繞射法估計之內應力範圍為：-6.204~-1.234GPa ，拉曼光譜法由於訊號穿透深度較淺，故內應力叫X-射線繞射法小，其範 圍為：-2.609~-0.729GPa。結果亦顯示本質應力的主要影響因素為鑽石膜 內（並非晶界中）非鑽石碳的比率。當CH4/H2濃度增加，沉積溫度降低均 會使薄膜內非鑽石碳含量增加，造成本質應力增加。當膜厚介於4-5um時 ，薄膜具有最小平均內應力；當膜厚介於10-12um時，會因產生二次結晶 現象使薄膜內非鑽石碳含量增高而具有最大平均內應力。 有關光學性質方面：實驗結果顯示鑽石膜之折射率於中紅外光區 （500-4000cm-1）隨波長及非鑽石碳含量而改變，其折射率平均範圍 為2.33-2.50，且較高的波長或較高的非鑽石碳含量均會造成較低的折射 率。光學穿透率主要受表面粗糙度及薄膜內非鑽石碳含量影響，膜厚對穿 透率的影響較小。當表面粗糙度控制在0.15-0.2um且膜厚介於10.8-23.6 um之間時，平均穿透率約42-48%，且厚度增加時，低波長範圍之CH- stretching吸收帶較為明顯，顯示薄膜具有較高的氫原子含量。實驗結果 亦顯示，內應力較低的薄膜具有較高的折射率。 Diamond films were deposited on P-type (100) silicon substrate by a hot filament chemical vapor deposition (HFCVD) system with reactant gases of CH4 and H2. Effects of CH4/H2 ratio, deposition time, deposition temperature and substrate pretreatment on residual stress and optical properties of films were investigated. The FTIR transmission spectra and their mathematical transmission modeling were used to analyze the infrared optical properties. The films were also characterized by SEM, Raman and XRD. The internal stresses of the films were estimated by Raman spectroscopy and X-ray diffraction, and the d-spacings of the crystal planes were rectified by the Rietveld analysis method. The purposes of this work were to investigate the optical properties and the origins of internal stress of diamond films, and to correlate the internal stress with the optical properties. The experimental results indicate that the origins of the residual stress of diamond films are mainly the thermal stress and the intrinsic stress. The internal stresses on Si(100) wafrs are ranging from -1.234 to -6.204 GPa, as determined by X- ray diffraction method, and from -0.729 to -2.609 GPa, as determined by Raman spectroscopy. The difference in stress level may be partly due to a limited penetration depth of laser beam in Raman spectroscope. The results also show that a higher non-diamond carbon content in the diamond crystal, not at the grain boundaries, will result in a higher compressive intrinsic stress. The deposition conditions which can cause an increase in non-diamond carbon content in the films, will result in an increase in compressive stress, such as, higher CH4/H2 ratios, or lower deposition temperatures, The results also indicate that the internal stresses at the thickness around 4-5um show a minimum value, and show a maximum value around 10-12um thickness. This is due to the fact that the maximum internal stress was companied by the renucleation phenomena, which result in an increase in non-diamond carbon content in the films. For optical properties, the results reveal that the refractive indexes of the films vary from 2.33 to 2.50, depending on wavelength and non-diamond carbon content. Alonger wavelength and a higher non-diamond carbon content will result in a lower refractive index. The optical transmittance of the films is depending mainly on the surface roughness and non-diamond carbon content, and slightly on the film thickness, and is ranging from 42-48% with the roughness of 0.15-0.2um and thickness 10.8-23.6um. The C-H stretching absorptions were found in the transmission spectra at higher thicknesses and lower wavelengths, indicating a high hydrogen content in the films. It is noted that a lower internal stress of the film will tend to have a higher refractive index.